The use of alkyl titanates as catalysts for the esterification of carboxylic acids as described by Weber, U.S. Pat. No. 3,056,818 is well known. The use of titanate chelates has been described by Deardorf in U.S. Pat. No. 4,526,725. A problem with the titanate catalysts is that the titanium residues must be removed, and typically this is done by treating the crude plasticizer with aqueous caustic followed by water washing and filtration to remove metal residues. This type of processing produces large amounts of waste water which must be disposed of and is obviously undesirable.
The use of alkali for the treatment of esters prepared in the presence of titanate catalysts is described in U.S. Pat. Nos. 4,007,218 and 3,818,071.
Deardorf in U.S. Pat. No. 4,506,091 uses steam at elevated temperatures to precipitate the titanium and the residues formed are removed by filtration. The use of steam also requires care in that ester hydrolysis can occur with a concomitant rise in the acid number. While steam treatment eliminates caustic treatment, it does not address the question of waste water.
Sagara et al, U.S. Pat. No. No. 4,284,793 describe the treatment of a titanate containing plasticizer with an excess of a powdered solid base, preferably NaHCO.sub.3 or Na.sub.2 CO.sub.3, at temperatures in the range of 100.degree.-200.degree. C. This type of treatment is exemplified in that no water is added to the diester. The solid alkali is removed for further use, and activated clay added with stirring. This is followed by the addition of diatomaceous earth, and finally both treatment agents are removed by filtration to give the product. Little waste water is generated by this procedure.
While the '793 patent is an improvement over other processes, it still suffers from the need to recover and store unused solid base, and from the use of two separate treating agents after the solid base has been removed. It also does not allow for the use of water in any of the treatment steps.